| Summary: | The sedimentary lithology is complex in the middle sub-member of the first member of the Liushagang Formation(L1-M) of the Weizhou-11A oilfield in the southwest Weizhou depression of the Beibu Gulf Basin. The reservoir changes rapidly in both vertical and horizontal directions, and the distribution scale and connectivity are challenging to predict, which seriously affects the oilfield development effect. Through core observation and description, 3D-seismic analysis, rock grain size, lithofacies, logging facies, and modern deposit analogy analysis, it is believed that the depositional fluids control the sand bodies and reservoir types. Braided stream bottom-load deposits (St, Sp, Sr), sandy debris flow deposits (Gmg, Gn), and turbidity deposits (Fhn) can form favorable reservoirs, which are mainly distributed in the braided channel, braided flow bar, mouth bar and sheet sand microfacies of fan delta front. We discussed the spatial distribution law and geometric shape of sedimentary microfacies in four middle-term cycles and proposed five types and eight kinds of sand-body stack types and connectivity characteristics of the fan delta front. The reservoir model and the empirical values of width, thickness, and the width-to-thickness ratio of main sedimentary microfacies are established, which provide a scientific basis for the oilfield development.
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